In the field of consolidation of composite products of lignocellulosic materials such as particleboard, hardboard, waferboard or plywood, it is known that by injecting and releasing steam into and out of the composite products during consolidation thereof using heat and pressure, several improvements are obtained, not the least of which is an increase of heat transfer rates that significantly speeds up the curing of thermosetting adhesives and results in lower swelling of the products. Several types of apparatus have been used for the introduction of steam into consolidation products.
Two general systems of steam injection pressing have been previously proposed, these being open pressing and closed pressing. In open steam injection pressing, steam is allowed to escape from the lignocellulosic board being formed through the open edges during pressing. Closed steam injection pressing, on the other hand, uses some type of a peripheral gasketing material or other means so that the steam is inhibited from escaping. Closed steam pressing is more efficient, but so far the design of a suitable gasket has proven illusive. Another problem with many of the prior art attempts is the use of massive platens, which substantially increases the capital cost, systems of this type being exemplified by the Corbin U.S. Pat. No. 3,280,257; Shen U.S. Pat. No. 3,891,738 and Nyberg U.S. Pat. No. 4,162,877.
For example, Shen U.S. Pat. No. 3,891,738 discloses a press platen which provides conventional internal heating by passing heating fluids through a closed labyrinth of interconnected passageways, and has an additional chamber and aperture openings on the surface adjacent to the product undergoing pressing through which steam is passed and injected into the product. In this arrangement, steam passes from a chamber of one press platen through the pressed product and from there into the opposite press platen, thereby speeding up curing of the thermosetting resins. Shen proposes a sealing frame carrying an asbestos gasket as a seal.
Nyberg U.S. Pat. No. 4,162,877 discloses the use of one platen which comes into contact with the product. Steam is injected from the press platen through the openings therein and into the product and is then released back through the same openings into the platen after curing of the thermosetting resin in the product. In the Nyberg construction, peripheral walls are used with built-in O-ring seals on the bottom sides of the walls, or attached on the inside of the wall siding along the press platens.
Futo U.S. Pat. No. 3,619,450 discloses a gas type envelope bellows consisting of Teflon sheet material suitably reinforced and sealed by a ring inserted between a metal ring and the platen. Clamps are required to secure the metal rings before steam injection takes place so as to ensure proper positioning and stability of the sealing ring.
The Hsu U.S. Pat. No. 4,850,849 employs steam injecting platens and border projection means generally configured to the outline of the end product and carried by the upper press platen which are intended to impinge and press into the mat at its edges to affect a steam pressure seal by excessive compression of the product along the periphery of the mat.
Stofko U.S. Pat. No. 4,409,170 discloses apparatus for forming porous products of low to medium density by consolidation of fibrous, particulate or laminar materials in the presence of a bonding agent under heat and pressure, comprising a press and at least one pair of pressing plates between which the consolidation of such materials is effected, including a peripheral seal surrounding the space between the pair of pressing plates when said plates are closed to press therebetween the materials to be consolidated to provide a closed and sealed pressing volume, at least one of the pair of pressing plates having horizontal permeability along its entire interiors and a vertical permeability along a central portion thereof, the pressing plate being flexible, thin, and of low mass and thermal capacity; and means to apply a fluid heat carrier to the interior of one of said pressing plates for passage of said heat carrier through the vertical permeability thereof.
Stofko U.S. Pat. No. 4,504,205 pertains to a press apparatus converted from a conduction press for consolidation of products by heat and pressure to a convection press therefor, comprising an upper press platen having a lower working face and a lower press platen having a upper working face; an upper caul plate attached to the lower working phase of the upper press platen; a lower caul plate on the upper working face of the lower press platen, each of said caul plates being formed with a central permeable area and a surrounding impermeable area along the peripheral edge thereof; sealing means for inhibiting fluid from escaping from the permeable areas of the caul plates; and steam feeding means provided in the upper and lower press plates for feeding steam from a source outside of the press platens to the central permeable area of said the plates.
It is known that metallic plates deform by uneven heating thereof and by imposing stresses upon them. Thick plates of high flexural rigidity (product of modulus of elasticity and moment of inertia of cross section) have the capability of resisting deformation, but these take up too much room in the opening of the press. Thin plates are easily deformed but also easily flattened by pressure. Plates of intermediate thickness deform and are not easily flattened. Pressures to which products are pressed are not sufficient to flatten them which results in uneven surface.
Another problem posed in the foregoing prior art closed steam injection pressing systems is the seal arrangement. The proposals of the aforementioned patents of Shen, Nyberg, Futo and the like for seals have not found practical application for several reasons, including complexity and difficulty of reliable sealing and easy usage. Because dependable seals have not yet been provided according to such prior art, closed steam pressing is not at present industrially used. Thus, it is extremely difficult to produce a perfect heat resistant seal along the periphery of a large press platen, which has an area of from 32 to 400 square feet, impermeable to gases of pressures up to 200 psi. In a closed steam injection pressing system, the seal has to be able to seal the space between press platens at variable thicknesses of products of from about one quarter to two inches. Therefore, the seal has to be very strong. For example, at a two inch distance between platens of about 4.times.8 feet area and 150 psi steam pressure, the force acting on inside surface of the seal is about 300 pounds for every inch of gasket length and this gives rise to a total force of about 9,600 pounds. Under such circumstances, the seal must remain flexible over a long period without fatigue and it must provide a tight seal under adverse conditions of particulate wood material, which is easily distributed over the area under the seal.